Topic: Cable Logic ? (for those who know more about E.E. than I do (most of you))

[Nick's Picks]

ok...
just a few quick thoughts.

I used to ground all of my cables to the housing of the XLRs, via the little pin that sticks out up top just for that purpose.  Every retail mic cable I've ever taken apart was built this way.
Recently, I was told that this is not a great practice, and that it can do more harm than good ...by passing on any grounding "buzzzz" or other interference.
Fact / fiction ? 

back to grounding...
it is common practice to use the shielding of a cable as the ground.  Why is this any better than just using a 3rd conductor (in a 3 wire setup) ?
As I understand it, the shielding is there to inhibit outside interference.  Why is grounding w/the shield better ?, does it help the shielding potential, or does it help by making a better ground ?

[ghellquist]

Myh two cents on this.

The general theory now is that you should ground the shell in XLR connectors. The idea is that nasty hum or interference should not be able to slink in through the connection. Personally I never connect the shield anymore. This after having trouble with some of my preamps. Seems like the shied was grounded there as well, and this created a ground loop with hum. Never had that problem anymore. To put it short, I am convinced it depends on the equipment used.

The shield should be grounded in order to protect from hum. And while it is, you might as well use it for the return path. If you run a three wire + shield cable, by all means use the third cable for ground, but connect the shield to ground as well. I would connect the shield to ground only in the male connector in order to avoid ground loops.

Edit: to be very clear. I always connect the shield in both ends. This has never been a problem with line levels for me. It also allows me to have only one type of cable, used both for phantom powered mics and line level.

Gunnar

[cshepherd]

This is from the DIY wiring diagrams at vandenhul.com

(http://www.vandenhul.com/artpap/wiring1.htm)



Drawn for line level connections; See remarks below.


Remarks:

    If the cable is to be used for microphone or non mains powered musical instrument connections, i.e. with low (non line level) signals, the cable shield should also be linked to the signal sending side connector’s ground pin. There are two reasons for this:

       1. To extend the shielding function to the microphone or instrument’s housing.
       2. And also with phantom powered condenser microphones: To provide a phantom power supply current return path.

    In all other cases (i.e. line level connections) leaving the cable shield disconnected at one side of the cable (drawn above) is safe practice. This to avoid ground loops with non properly internally grounded equipment.
    If with these “one side disconnected shield” connections RF interference problems are experienced, reconnecting the shield at the side where it was disconnected by means of a series network of a 47 Ohm resistor and a 10 nanoFarad ceramic disc capacitor is advised. Latter may be standardly applied for all balanced line level interconnections.
    Reference: K.R. Fause. Fundamentals of Grounding, Shielding and Interconnection. Journal of the Audio Engineering Society, Vol. 43, No. 6, June 1995, p. 498-516.

    Note: Van den Hul ready made balanced cables standardly have the cable shield connected at both ends allowing correct functionality with microphones and electrical instruments as well as with line level audio and AES/EBU digital connections, latter two assuming properly grounded equipment.

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Edit to Add:  van den Hul Mic cable has three conductors / three shields.  The factory-terminated set we originally purchased had the braided, outer shield joining the third conductor on the ground pin at both ends.

Chris

[Nick's Picks]

thanks guys....

so the general consensus is:

ground the shield on the male end, but not the female.

when using 3 wire, use one of them as ground, but also ground the shield (which will make it grounded on both ends).

that sort of opposes the first part about not grounding the female end.

[cshepherd]

Hi Nick,
The way I read it, vdH recommends connecting the screen at both connectors' ground pin when the cable is going to be used as a (low level) microphone or instrument cable - for two reasons:

1. To extend the shielding function to the microphone or instrument’s housing.  (Low level signals are more susceptible to outside EMI / RF interference than line-level signals)
2. And also with phantom powered condenser microphones: To provide a phantom power supply current return path.

If it's going to be used as a line level interconnect, it's common to leave the screen disconnected at the female connector to avoid ground loops when used with gear that doesn't have proper internal grounding.  If ground loops aren't a problem for the gear being used with the interconnect, it's safe to connect the screen at both connectors for added EMI / RF interference protection (which is how vdH termiinates all xlr cables) Keep in mind that EMI / RF interference negatively affects the signal before audible interference noise comes through.  IMO, the more the better when it comes to EMI / RF interference protection.

Chris

[Nick's Picks]

that makes a little more sense to me.
Thanks Chris.  +t

[sanaka]

Here is how I understand a few items:

A balanced mic cable wants all three XLR pins on both ends.

If the pin 1's are connected using the cable's "shield" - the braided or foil conductive material that makes a sheath around the group of insulated conductors - then that shield is doing double duty: acting as signal ground as well as the RFI shield. This is how it's usually done.

If you have three conductors in the cable (which seems cool, I'd like to see that cable!), then you can use one of them to connect the pin 1's for signal ground. In this case you can do what you wish with the surrounding shield material. You can connect it at both ends as usual, or if it is connected at only one end (usually the signal downstream end), then it is strictly an RFI shield, not carrying signal ground. In either case the connection point(s) of the shield is(are) soldered at the same terminal(s) as the signal ground.

When an XLR connector is plugged into a device, the connector's shell may be tied to the device's equipment ground, an entirely different thing than signal ground. Various devices, especially mains powered ones, implement many different equipment grounding schemes, some of which, if tied to signal ground, make bad things happen to the signal. In general it is therefore advisable to keep equipment and signal ground paths separate. That is why not to solder your shield to the XLR shell.

But I waste your time. Read it from the horse's mouth here.

Peace,
Sanaka

[cshepherd]

If you have three conductors in the cable (which seems cool, I'd like to see that cable!)...

Peace,
Sanaka

http://taperssection.com/index.php/topic,77224.0.html

Chris

[sanaka]

Wow!